Fibrous materials are well known for their use as thermal insulating materials and are also known for their use as strengthening constituents in composite materials such as, for example, fibre reinforced cements, fibre reinforced plastics, and as a component of metal matrix composites.
Prior to 1987 there were four principle types of fibrous materials used for making thermal insulation products [such as, for example, blanket, vacuum formed shapes, and mastics]. These were made by two principal manufacturing routes although the details of the particular routes vary according to manufacturer. The fibres and routes were (in order of increasing cost and temperature performance):    Melt formed fibres            Glass wools        Mineral wools        Aluminosilicate fibres            Sol-gel process fibres            So-called polycrystalline fibres        
Because of the history of asbestos fibres, a lot of attention has been paid to the relative potency of a wide range of fibre types as a cause of lung disease. Studies of the toxicology of natural and man-made fibres led to the idea that it was the persistence of fibres in the lung that caused problems. Accordingly, the view developed that if fibres can be removed from the lung quickly then any risk to health would be minimised. The concepts of “biopersistent fibres” and “biopersistence” arose—fibres that last for a long time in the animal body are considered biopersistent and the relative time that fibres remain in the animal body is known as biopersistence. Whilst several glass systems were known to be soluble in lung fluids, resulting in low biopersistence, there was a problem in that such glass systems were generally not useful for high temperature applications. A market need was seen for a fibre that could have a low biopersistence combined with a high temperature capability. In 1987 Johns Manville developed such a system based on a calcium magnesium silicate chemistry. Such material not only had a higher temperature capability than traditional glass wools, but also had a higher solubility in body fluids than the aluminosilicate fibres mostly used for high temperature insulation. Such low biopersistent fibres have been developed since, and a range of alkaline earth silicate [AES] fibres are now on the market. Patents relating to AES fibres include:                International Patent Application No. WO87/05007—the original Johns-Manville application—which disclosed that fibres comprising magnesia, silica, calcia and less than 10 wt % alumina are soluble in saline solution. The solubilities of the fibres disclosed were in terms of parts per million of silicon (extracted from the silica containing material of the fibre) present in a saline solution after 5 hours of exposure.        International Patent Application No. WO89/12032 disclosed additional fibres soluble in saline solution and discussed some of the constituents that may be present in such fibres.        European Patent Application No. 0399320 disclosed glass fibres having a high physiological solubility and having 10-20 mol % Na2O and 0-5 mol % K2O. Although these fibres were shown to be physiologically soluble their maximum use temperature was not indicated.        
Further patent specifications disclosing selection of fibres for their saline solubility include for example European 0412878 and 0459897, French 2662687 and 2662688, WO86/04807, WO90/02713, WO92/09536, WO93/22251, WO93/15028, WO94/15883, WO97/16386, WO2003/059835 WO2003/060016, EP1323687, WO2005/000754, WO2005/000971, and U.S. Pat. No. 5,250,488.
The refractorness of the fibres disclosed in these various prior art documents varies considerably and for these alkaline earth silicate materials the properties are critically dependent upon composition.
As a generality, it is relatively easy to produce alkaline earth silicate fibres that perform well at low temperatures, since for low temperature use one can provide additives such as boron oxide to ensure good fiberisation and vary the amounts of the components to suit desired material properties. However, as one seeks to raise the refractoriness of alkaline earth silicate fibres, one is forced to reduce the use of additives since in general (albeit with exceptions) the more components are present, the lower the refractoriness.
WO93/15028 disclosed fibres comprising CaO, MgO, SiO2, and optionally ZrO2 as principal constituents. Such AES fibres are also known as CMS (calcium magnesium silicate) or CMZS ((calcium magnesium zirconium silicate) fibres. WO93/15028 required that the compositions used should be essentially free of alkali metal oxides. Amounts of up to 0.65 wt % were shown to be acceptable for materials suitable for use as insulation at 1000° C.
WO94/15883 disclosed a number of such fibres usable as refractory insulation at temperatures of up to 1260° C. or more. As with WO93/15028, this patent required that the alkali metal oxide content should be kept low, but indicated that some alkaline earth silicate fibres could tolerate higher levels of alkali metal oxide than others. However, levels of 0.3% and 0.4% by weight Na2O were suspected of causing increased shrinkage in materials for use as insulation at 1260° C.
WO97/16386 disclosed fibres usable as refractory insulation at temperatures of up to 1260° C. or more. These fibres comprised MgO, SiO2, and optionally ZrO2 as principal constituents. These fibres are stated to require substantially no alkali metal oxides other than as trace impurites (present at levels of hundredths of a percent at most calculated as alkali metal oxide). The fibres have a general composition                SiO265-86%        MgO 14-35%with the components MgO and SiO2 comprising at least 82.5% by weight of the fibre, the balance being named constituents and viscosity modifiers.        
WO2003/059835 discloses certain calcium silicate fibres in which La2O3 or other lanthanide additives are used to improve the strength of the fibres and blanket made from the fibres. This patent application does not mention alkali metal oxide levels, but amounts in the region of ˜0.5 wt % were disclosed in fibres intended for use as insulation at up to 1260° C. or more.
Such fibres are made from the melt by forming a molten stream and converting the stream into fibre either by permitting the stream to contact a spinning wheel, or by using an air blast directed at the stream. Features of such melt formed fibres include:                since the rapid changes in temperature during the forming process results in a rapid change of viscosity, the fibres come in a wide range of diameters        the fibres have a much lower strength than might be expected from the bulk properties of the fibre composition, the applicants suspect due to the introduction of flaws in the fibre forming process        a large amount of shot (unfiberised material) is normal for such materials—typically >40% by weight comprises shot—the presence of shot raises thermal conductivity of fibrous insulation materials—although fibre can be deshotted this adds to expense.        
Also, the scope of such low biopersistence fibres is limited in that above about 1300° C. they tend to deteriorate in performance. Further, for some applications the mechanical properties of such fibres are not adequate. As an example, most modern vehicles are equipped with pollution control devices such as catalytic converters or diesel particulate filters. Such pollution control devices typically comprise a treated monolithic ceramic structure (typically a honeycomb construction) used to purify exhaust gases at high temperatures and secured within a metal housing by a resilient and flexible mat that is typically formed from inorganic fibres. Exhaust gases enter one end of the control device, where the gasses are treated, and exit the other end.
Such exhaust catalytic converters and diesel particulate filters require fibres that will maintain their compressive strength and resilience over a wide range of temperatures. [By resilience, in this context, is meant the ability of an article to recover its initial shape after deformation]. To cope with the high temperatures (typically 850° C.-950° C. for present catalytic converters) and constant thermal cycling encountered in such devices requires fibrous material that has a high degree of resilience to provide a support to the fragile catalytic structure. At present catalytic converters use either aluminosilicate fibres that have been heat treated to provide the appropriate degree of resilience, or sol-gel formed alumina and/or mullite fibres. The problem is however that both aluminosilicate fibres and such sol-gel formed fibres have low solubility in siulated body fluids, and are expected to have high biopersistence compared with AES fibres. There are no known low biopersistence fibres that are suitable for use in such pollution control devices. This is of concern, since the large number of such pollution control devices and their widespread use gives a great opportunity for exposure to the fibres.
Alternative low biopersistence fibres that have been proposed are alkaline earth aluminates. Such materials have been suggested as calcium aluminate (EP0586797) and strontium aluminate (WO96/04214). Such fibres are not produced commercially, but as they are described as formed from a melt they would have the same characteristic variability in fibre diameter and high shot content.
Vitreous fibres such as melt formed silicate fibres are subject of regulation in Europe, and different fibre classes have different hazard classifications and labelling requirements. Conventional vitreous alumino-silicate fibres require more stringent labelling concerning health hazards [as so-called category 2 carcinogens] than do alkaline earth silicate fibres which are exonerated from carcinogen classification. Sol-gel polycrystalline fibres are not, as yet the subject of hazard classification in Europe.
Conventional vitreous fibre processing techniques used for the production of alkaline earth silicate fibres discussed above require conversion of the raw materials into a homogeneous high temperature melt and subsequent fiberisation of the melt.
In sol-gel fibre processing, a sol is formed from precursor materials. Fibrous gels are formed from the sols (generally at around room temperature), and then are converted to glass or ceramic fibres by heating at elevated temperatures (e.g. 700° C. to 2000° C.). Various kinds of fibres have been prepared by this type of sol-gel technique (e.g. silica, alumina-silica, zirconia, alumina and titania).
Sol-gel formation of fibres has the advantages over melt forming that:    a) melt forming becomes progressively more difficult as the temperature of the melt increases and uncontrolled crystallisation can occur    b) as higher melt temperatures are required it becomes difficult to find materials for the apparatus that have a reasonable working life at the temperatures involved    c) sol-gel techniques enable the production of materials where the components are insoluble or immiscible in the melt.
Sol-gel formed fibres tend to have a lower (but not zero) shot content in comparison with melt formed fibres. Known refractory sol-gel formed fibres have a range of compositions ranging from, for example, mullite fibres of a general composition 3Al2O3.2SiO2 through to fibres that are almost pure Al2O3. Examples include products under the trade names:                MAFTEC™ a fibre produced by Mitsubishi Chemical Corporation and which comprises ˜72% by weight Al2O3 and 28% by weight SiO2, and alleged to have a mean diameter in the region of 4 μm.        SAFFIL™ a fibre produced by Saffil Limited and which comprises ˜96-97% by weight Al2O3 and 3-4% by weight SiO2, with trace elements <0.5%, and alleged to have a mean diameter in the region 3-4 μm.        NEXTEL™ a fibre produced by 3M and which has a range of compositions from ˜62% by weight Al2O3, 24% by weight SiO2, and ˜14% by weight B2O3 through to >99% by weight Al2O3, 0.2-0.3 by weight SiO2 and 0.4-0.7% Fe2O3. Nextel fibres are alleged to have a typical mean diameter of 10-12 μm.        FIBERMAX™ a fibre produced by Toshiba Monofrax and Unifrax Corporation, and which comprises ˜72% by weight Al2O3, 27% by weight SiO2, and ˜1% by weight other components including very small quantities of MgO, CaO and Na2O [<0.2% each], and alleged to have a mean fibre diameter in the region of 2-3.5 μm.all of which contain various proportions of SiO2 and Al2O3. The relatively high degree of resilience of sol-gel formed fibres in comparison to alkaline earth silicate fibres at temperatures in excess of about 1300° C. makes them ideally suited to catalytic converters. For example, a few of the many patents describing the use of such sol-gel formed fibres in catalytic converters include; U.S. Pat. Nos. 4,929,429, 5,028,397, 5,032,441, 5,580,532, 5,666,726, 5,811,063, 6,726,884 WO00/75496 and WO2004/064996.        
Biopersistence is not the sole factor in ascertaining the potential health hazards of fibrous materials. Also of relevance is the amount of fibre that is respirable. If a fibre does not enter the lung it cannot cause damage to the lung. Sol-gel fibre techniques permit the production of fibres having a relatively narrow fibre diameter distribution, and the argument of sol-gel fibre manufacturers is that this enables the reduction in the proportion of respirable fibres from their materials. Reduction is not the same as elimination however, and the production of a sol-gel fibre that has acceptable mechanical and thermal properties and that is soluble in physiological saline solutions offers the opportunity of not only limiting the amount of respirable fibre, but also ensuring that what fibre is respirable has a reduced biopersistence in comparison with conventional sol-gel fibres.
The applicant has discovered that it is possible to produce sol-gel formed fibres that exhibit a low shrinkage at elevated temperature, and a high resilience at temperature, and that also have the virtue of having a degree of solubility in body fluids that while not as high as the best alkaline earth silicate fibres, is significantly higher than the solubility of pure mullite fibres. There is a trade-off in these requirements and the present invention permits the production of highly refractory—slightly soluble materials at one extreme to very soluble—reasonably refractory materials at the other with a range of characteristics in between.
U.S. Pat. No. 5,019,293 discloses methods of making magnesium aluminium silicate sol-gel fibres in which the ratio of Mg to Si ranges from 0.3:1 to 4:1 and the ratio of Mg to Al ranges from 0.12:1 to 2:1. The method involved comprises the manufacture of a low concentration sol [<1% solids] using hydrogen peroxide in the sol forming process, concentrating the sol, and forming fibre by:                extruding the sol into a basic solution        extruding the sol into air and onto a coated substrate        coating a string or filament with sol drying the fibres then takes 8 hours to 3 days and the fibres are short [1 mm to 2 cm] and with aspect ratios of 50 to 200 this implies fibre thicknesses in the range 5-400 μm. Such fibres would not be considered as useful for insulation purposes.        
JP59082412 discloses sol-gel fibres comprising <6 wt % [˜14 mol %] MgO which are indicated in the abstract as having improved flexibility. A comparative example with 10 wt % [˜22 mol %] MgO was indicated as being unsatisfactory. No mention is made in the abstract of the use of these fibres as thermal insulation.
U.S. Pat. No. 3,982,955 discloses alumino-silicate sol-gel fibres comprising 0-5% MgO.
U.S. Pat. No. 4,010,233 discloses MgO.Al2O3 sol-gel fibres.